1. Field of the Invention
This invention relates to thixotropic organosiloxane compositions. More particularly, this invention relates to multi-part moisture curable organosiloxane compositions containing specified classes of thixotropic agents and fillers. The thixotropic agent, crosslinking agent, crosslinking catalyst and a specified class of solvents for the thixotropic agent are packaged separately from the crosslinkable polyorganosiloxane and a hydroxyl-containing filler.
2. Background Information
Various materials have been used to impart thixotropy to curable organosiloxane compositions.
U.S. Pat. No. 5,036,131, which issued to A. Himstedt on Jul. 30, 1991 describes silicone dispersions containing 100 parts by weight of a liquid silanol-terminated polydimethylsiloxane, a moisture activated curing agent, and finely ground alumina trihydrate. The thixotropic agent is the combination of an untreated fume silica and a liquid silanol-terminated polydiorganosiloxane containing phenylmethylsiloxane or 3,3,3-trifluoropropylsiloxane units that is incompatible with the polydimethylsiloxane. The polydiorganosiloxane reacts with the silica to form a surface that is incompatible with the polydimethylsiloxane.
From 0.25 to 2.5 part by weight of the incompatible polydiorganosiloxane is used per part of untreated silica.
Russian patent 1,502,586 issued to A. Chuiko et al. describes protective coatings containing a polymethylphenylsiloxane, toluene and a pyrogenic form of silica that has been surface-modified with methyl groups.
A disadvantage of using the incompatible filler described in the patents to Himstedt and Chuiko et al. in the base portion of a two part moisture curable organosiloxane composition comprising a silane containing alkoxy or other hydrolyzable groups as the curing agent and an organotin compound as the curing catalyst is the difficulty of processing the resultant highly thixotropic material.
East German patent No. 297,178, which issued to E. Alekseewa et al. on Jan. 2, 1992 describes compositions comprising a polyorganosiloxane containing methyl, phenyl and vinyl radicals, an organohydrogensiloxane containing methyl and phenyl radicals, and a platinum catalyst. The compositions also contain as a thixotropic agent a resinous copolymer with a) an organosiloxane copolymer containing units of the formulae SiO.sub.4/2, (b) RSiO.sub.1.5, (c) R.sub.2 SiO and (d) R.sub.3 SiO.sub.0.5 units, wherein from 5 to 30 mole percent of the R groups are phenyl, 1 to 5 mole percent are vinyl, the remainder are methyl and the molar ratio (c+d):(a+b) is from 0.7 to 1.4.
Other thixotropic agents that have been used in curable organosiloxane compositions include silica treated with various organosilicon compounds, chalk, dolomite, and organosiloxane/oxyalkylene copolymers in combination with a filler containing hydroxyl groups.
To achieve adequate storage stability some moisture curable organosiloxane compositions, particularly those containing tin compounds as the catalysts for the crosslinking reaction, are packaged with the tin compound and crosslinkable polyorganosiloxane in separate containers. The crosslinking agent is typically packaged together with the catalyst.
Because only a small volume of crosslinking agent relative to the volume of crosslinkable polyorganosiloxane is required to achieve the desired degree of crosslinking, the part of the composition containing the crosslinking agent typically contains a quantity of diluent that will allow this part to be blended with the part containing the crosslinkable polyorganosiloxane in a volume ratio of 1 part of curing agent portion to at least 10 parts of the portion containing the crosslinkable polyorganosiloxane. The diluent is a solvent for the crosslinking agent and the catalyst.
For some end use applications of two-part moisture curable organosiloxanes thixotropy would be undesirable. If the thixotropic agent were in the portion of the composition containing the crosslinkable polyorganosiloxane, typically referred to as the "base" portion, a user who wished to alternate between thixotropic and non-thixotropic compositions using the same mixing equipment for both types of compositions would have to change the container of base. If the thixotropic agent was in the crosslinking agent portion, using the conventional volume ratio of base to crosslinking agent portion of 10:1, the volume of material to be transferred during the change from a thixotropic to a non-thixotropic composition would be 1/10 of the volume to be transferred if the thixotropic agent was in the base portion.
The present inventors discovered that many of the diluents used in the crosslinking agent portion of conventional two-part moisture curable organosiloxane compositions will not dissolve the organosiloxane/oxyalkylene copolymers used as thixotropic agents in these compositions. The resultant two-phase composition results in non-uniform curing of the composition.
One objective of this invention is to provide multi-part moisture curable organosiloxane composition wherein the portion containing the crosslinking agent also contains a solubilized siloxane/oxyalkylene copolymer as the thixotropic agent. The resultant mixture is compatible with the crosslinkable polyorganosiloxane contained in the base portion of the composition, and forms a homogeneous composition when all the ingredients of the curable composition are combined.